The efficiency of a turbocharger on a diesel engine has been an important design consideration for many years particularly with the trend towards the diesel engine being subjected to higher torque rise and lower torque peak speeds. A turbine casing is essentially made up of a volute-shaped conical section wrapped around a turbine wheel. Analyses have been based upon the decreasing area or decreasing A/R (area.div.radius) around the circumference of the casing. Using these conventional methods, either the cross-sectional area of the volute-shaped passageway or the A/R value at any tangential location decreases uniformly through an angle of 360.degree.. These methods are described in the literature and are well known to those skilled in the art of turbine design.
To meet the efficiency and operating requirements described above, various types of turbine casings of both fixed and variable geometry have heretofore been developed; however, such casings have been beset with one or more of the following shortcomings: (a) the casing was of complex, costly and bulky construction; (b) the vortex of the passageway did not remain centered with respect to the turbine wheel, resulting in non-uniform wheel boarding states and exit states around the periphery of the turbine exducer; (c) the turbine wheel's pressure ratio versus mass flow characteristics were not matched to minimize wheel exit losses; (d) turbine wheel blade vibration was excessive, leading to turbine wheel mechanical failures; and (e) the percentage of change in the width of the passageway did not remain substantially constant throughout the length of the passageway. Additional problems included fluid mixing problems near the housing tongue, and angular momentum losses in the housing and turbine.